Some Escherichia coli carry plasmids which confer on the host bacterium the ability: 1) to produce an antibiotic-like protein called colicin whose adsorption to specific bacterial receptors leads to cell death, and 2) to survive treatment with homologous colicin, called immunity. To further our understanding of colicin action, we have undertaken a program:1) to study the events associated with induced synthesis of colicin 1, 2) to isolate and characterize the E. coli colicin 1 specific receptor as well as to study its interaction with colicins, and 3) to correlate the structure of colicins with their biological specificities. Our major effort at the current time involves studies related to the membrane protein which serves as the colicin 1 receptor. We are studying both the mechanisms involved in the regulation of receptor biosynthesis as well as the molecular events accompanying the insertion of this protein into the bacterial envelope.